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Fuel treatment effects on forest carbon and wildfire

Fuel treatment effects on forest carbon and wildfire. Malcolm North, Sierra Nevada Research Center, mpnorth@ucdavis.edu. By one estimate annual forest growth can offset 6-10% of anthropogenic CO 2 But these gains can be offset by emissions in fire-prone forests.

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Fuel treatment effects on forest carbon and wildfire

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  1. Fuel treatment effects on forest carbon and wildfire Malcolm North, Sierra Nevada Research Center, mpnorth@ucdavis.edu

  2. By one estimate annual forest growth can offset 6-10% of anthropogenic CO2 But these gains can be offset by emissions in fire-prone forests Wildfires: A Large Source of Emissions (Wiedinmyer and Neff 2007)

  3. Premises: • In California’s fire-dependent forests, significant C can be released during wildfire • The amount of that release increases with fire severity and size • General objective: If possible increase C storage, reduce the risk of C loss due to wildfire and in the process minimize C emissions • Forests need to be managed for more than just carbon sequestration • Fortunately, forest restoration and C management share a common long-term objective: Redirect ecosystem C away from unstable (the growth of numerous, small trees) to stable pools ( the growth of fewer, large-size trees which are fire-resistance [i.e., pines]) • The question is how to get there and what are the tradeoffs between different means to that end

  4. Conceptual Model of Tradeoffs in Fire-Dependent Forests Low High ‘Risk’ Future What we know: The general shape of these curves Present ‘Benefits’ * Effect % Desired direction What we don’t know: The slope and inflection points How these vary by forest type, productivity, etc. * If Ladder AND Surface fuels are removed

  5. Potential for Increasing Forest C Storage Less carbon in modern fire-suppressed forests than active-fire (1865) forests due to loss of large trees The plus is forests have potential to sequester a lot more carbon Total Live Tree Carbon Stocks: 1865: 346 Mg C/ha Current Forest: 249 Mg C/ha >

  6. Model estimate of wildfire emission is 38 Mg C/ha Bonnicksen estimated 156 Mg C/ha average for CA wildfires (FCEM Report #2) A forest structure (1865) of a low density of large pines has lowest wildfire and prescribed fire emissions

  7. Untreated Fire Direction Thinned and Prescribe Burned

  8. a) control c) understory thin e) overstory thin Live tree C and diameter distribution by species before and after 6 treatments Desired: higher C stocks, greater density and a higher % of pine in the large dbh classes > > > 1865 Control Pre-treat Post-treat Pre-treat Post-treat d) understory thin/burn b) burn only f) overstory thin/burn > > > Pre-treat Post-treat Pre-treat Post-treat Pre-treat Post-treat

  9. Understory thin 1865 Understory thin/burn Control Overstory thin/burn Overstory thin Burn only

  10. Summary: What we may know and what we clearly don’t know • Fuels treatments: • Reducing ladder AND surface fuels reduces fire severity. • Reducing surface fuels is key to restoring many ecosystem processes • Thinning overstory trees (reducing crown bulk density) has a limited effect on reducing fire severity. • Simulations (field data is absent) suggest treating 20-30% of a landscape can significantly reduce fire severity and extent • Don’t know: How long fuel treatments remain effective • How treatments affect residual tree growth (rate of C sequestration) • When reducing crown fire risk, how resistant should the forest be made? (i.e. what percentile weather conditions should be targeted? Impacts of climate change?)

  11. Summary: What we may know and what we clearly don’t know • Carbon Dynamics: • Within the limits of current measurements: 60-70% of ecosystem C is above ground, with about 80% of that in live trees. • In uncut forests, trees 5-25 cm dbh and 25-50 cm generally will contain about 5% and 15-20%, respectively of total live tree C. • Fuels treatments reduce forest C, losses increase exponentially with tree size • In most fuels treatments, fossil fuel use is a small % of C loss. • C losses from milling waste and prescribed burn vary but are probably in the range of 5-20% of aboveground C • Estimates of prescribed fire emissions (15-25 Mg C/ha) are probably within the actual ‘ballpark’ but wildfire emissions (25-45 Mg C/ha) , while improving, are still very rough. • Estimates of fire CO2 emissions are hampered by our lack of knowledge about C deposition, rates of atmospheric vs. soil incorporation of dead wood C, ‘real’ soil C loss, etc.

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